1. Field of the Technology
The invention relates generally to mobile communication technology, especially to a correction method for a time varying channel in a time-slot division mobile communication system. The invention provides an iterative correction method for channel estimation, applicable to a time-slotted CDMA system or other, similar block processing systems.
2. Related Technology
Complexity and time varying characters of channels are the most obvious characteristics of wireless mobile communications. In a coherent receiving system, it is necessary to estimate and measure channels at a receiving end, and then perform coherent detection with the obtained channel responses.
Channel response is determined by characteristics of a transmitting device, propagation of RF carrier in a wireless channel, and a receiving device. Doppler frequency shift caused by a moving terminal, a radio propagation channel response variations, fading variations, frequency drift, and deviation of devices etc. will lead to a time varying channel response and eventually affect performance of a system. Therefore, at the receiving end, the channel estimation and measurement must keep up with the channel variation in order to guarantee a good demodulation performance for received data.
In the time-slotted CDMA system or a similar block processing system, usually it is supposed that within a block (a time-slot or a burst signal), the channel response is time invariant. The assumption and other limitations bring great convenience to system design and relevant processing. Nevertheless, the assumption that the channel response is time invariant within a block has limited the application of the system, i.e. the application thereof has high requirements on frequency deviation and shift as well as on the moving speed of a terminal.
The burst structure in a traffic time-slot in the TD-SCDMA (3GPP 1.28 Mcps TDD) system is taken as an example for further description. As shown in FIG. 1, a midamble at the middle of a burst signal is used for channel estimation and can be called a channel estimation code while at both sides of the midamble are traffic data fields. In the TD-SCDMA system, the parameters of a traffic time-slot are as follows: the time-slot length is 675 μs; the length of a symbol is 12.5 μs when the spread factor is 16; and the length of a chip is 781 ns. Each time-slot (or a burst signal) has two traffic data fields, each of which has 22 symbols (spread factor 16). Thus each of the two traffic data fields has a total of 352 chips and a midamble of 144 (128+16) chips.
If a channel response is time invariant in a time slot, then channel response estimates for each user in this time slot can be obtained with the midamble. Carrying out joint detection (or matched filtering) and demodulation for the two traffic data fields with this fixed channel response estimation result, respectively, and data from the transmitting end can be recovered at the receiving end. This block processing mode brings convenience to system design, channel estimation and signal detection at a receiving end, making it possible for new techniques such as multi-user channel estimation and signal joint detection to be applied practically. Refer to the following relevant documents: [1] A. Klein, G. K. Kaleh and P. W. Baier, “Zero forcing and minimum mean square error equalization for multiuser detection in code division multiple access channels,” IEEE Trans. Veh. Tech., vol. 45, pp. 276–287, May 1996; [2] M. Vollmer, M. Haardt, and J. Gotze, “Comparative study of joint-detection techniques for TD-CDMA based mobile radio systems,” IEEE J Select. Areas Commun., vol. 19, no. 8, pp. 1461–1475, August 2001.
Nevertheless, the block processing method limits the system working conditions. When the channel response varies within a time-slot, the system performance will obviously become worse. Phase variation caused by channel response variations will worsen the performance of demodulation that contains phase, modulation signals, such as phase shift keying (PSK) and quadrature amplitude modulation (QAM); and an amplitude variation caused by channel response variation will worsen the performance of demodulation that contains amplitude modulation signals, such as QAM. The main causes that lead to channel response variation are as follows: deviation between transmitting frequency and receiving frequency; Doppler frequency shift caused by a moving terminal and fast fading etc. For example, with the 2 GHz carrier frequency, the maximum Doppler frequency shift is 463 Hz when the terminal moving speed is 250 km/h, the phase varying at one of the two ends of a TD-SCDMA time-slot relative to the phase at the center of the time-slot could be up to 1.25×π/4, making it impossible to demodulate a QPSK signal. The limitation will become more strict when 8-PSK modulation or 16-QAM modulation is used.
It is seen from the foregoing example that the system performance will deteriorate in a mobile communication system based on block processing when the channel varies quickly. Therefore, when it is required that the channel response does not vary within one time-slot, application of the system will be strictly limited, and the limitation is mainly on frequency deviation and shift as well as on the moving speed of a terminal.
Therefore, in a time-slotted CDMA system or other similar block processing systems, it is very significant to improve the system performance in case of quickly varying channels. In other words, it is necessary to correct the channel response variation within a block for the block processing system. However, there are two difficulties in performing the correction.
One is complexity of the processing: if the time invariant characteristic within a block is destroyed, the processing complexity will be increased sharply, which will create practical difficulties.
The other is the difficulty in obtaining the channel response variation: if only one channel estimation is given in one time-slot, and the two neighboring time-slots of a user are not consecutive, such as the time division duplex (TDD) system in 3GPP, then it is impossible to know the channel response varying situation from channel estimation results through interpolation and prediction.
In summary, in the time slot CDMA or similar systems, when processing burst data or time-slot data as a block with the channel response being time invariant, system design as well as channel estimation and signal detection at the receiving end will become convenient because techniques, such as multi-user channel estimation and joint detection, can be practically applied. Nevertheless, in systems with this block processing method, there are more strict limitations on channel response variations caused by factors such as frequency deviation and shift as well as terminals moving at a high speed. When channel characteristics are varied within a block, the performance of a system based on block processing will deteriorate. As a result, the block processing method limits the application of the system by limiting the scope of frequency deviation and shift in the system as well as the moving speed of a terminal.